307 
acters of peroxidised pyrites. The flakes of mica do not 
lie in the plane of bedding, but are inclined tolerably evenly 
at all angles, so that there is no definite line of structural 
weakness, independent of that due to bedding, which results 
chiefly from alternations of layers of somewhat different 
composition, and not from the arrangement of the ultimate 
particles. This is, however, totally different in a rock of 
similar composition having cleavage. If a section be ex- 
amined cut perpendicular to cleavage, in the line of its dip, 
it will be seen that though some of the minute flakes of 
mica lie perpendicular to the cleavage, or at high angles to 
it, by far the larger part are inclined at low angles, so that the 
majority lie within 20° on each side of it. In fact they are 
most numerous nearly in the plane of cleavage, and gra- 
dually but rapidly diminish in quantity in passing to higher 
angles, so that there are twenty times as many nearly in 
the plane of cleavage as at 45° to it, and very few at 90°. 
Where a section is examined cut perpendicular to cleavage 
in the line of the strike, it is seen that the arrangement is 
similar, but there is not nearly so rapid a diminution of the 
numbers in passing from the line of cleavage, so that there 
are comparatively several times as many more inclined at 
about 45° to it, as when the section is in the line of dip, 
and those at still higher angles are also much more nume- 
rous. In a section in the plane of cleavage, but few flakes 
are cut through so as to have a greatly unequiaxed form ; 
but they are similarly arranged with respect to the line of 
dip, though not in so marked a manner. It is not merely 
the larger flakes of mica that are thus arranged, but the 
whole of those unequiaxed particles which existed in the 
rock before the cleavage was developed. 
When a cleavage crack in the thin sections is examined, 
it is clearly seen that the cleavage is due to the above- 
described arrangement of the particles, which it follows 
